Rock lichens are like snowflakes.

Super-snowflakes.

Lichen on rock, Mud Bay Station Park

I've come to that conclusion after the better part of an hour searching through photographs of crustose lichens on rocks.

They're even more like snowflakes than the snowflakes themselves; snow at least comes in fixed geometric shapes, and is usually white. Among the lichen, there are no two alike, according to any criteria you may name. Except that the crustose ones lie flat.